sql 、linq、lambda 查询语句的区别
LINQ的书写格式如下：
from 临时变量 in 集合对象或数据库对象
where 条件表达式
[order by条件]
select 临时变量中被查询的值
[group by 条件]
Lambda表达式的书写格式如下：
(参数列表) => 表达式或者语句块
其中： 参数个数：可以有多个参数，一个参数，或者无参数。
参数类型：可以隐式或者显式定义。
表达式或者语句块：这部分就是我们平常写函数的实现部分（函数体）。
1.查询全部
1 查询Student表的所有记录。
2 select * from student
3 Linq:
4 from s in Students
5 select s
6 Lambda:
7 Students.Select( s => s)
2 按条件查询全部：
1 查询Student表中的所有记录的Sname、Ssex和Class列。
2 select sname,ssex,class from student
3 Linq:
4 from s in Students
5 select new {
6 s.SNAME,
7 s.SSEX,
8 s.CLASS
9 }
10 Lambda:
11 Students.Select( s => new {
12 SNAME = s.SNAME,SSEX = s.SSEX,CLASS = s.CLASS
13 })
3.distinct 去掉重复的
1 查询教师所有的单位即不重复的Depart列。
2 select distinct depart from teacher
3 Linq:
4 from t in Teachers.Distinct()
5 select t.DEPART
6 Lambda:
7 Teachers.Distinct().Select( t => t.DEPART)
4.连接查询 between and
1 查询Score表中成绩在60到80之间的所有记录。
2 select * from score where degree between 60 and 80
3 Linq:
4 from s in Scores
5 where s.DEGREE >= 60 && s.DEGREE < 80
6 select s
7 Lambda:
8 Scores.Where(
9 s => (
10 s.DEGREE >= 60 && s.DEGREE < 80
11 )
12 )
5.在范围内筛选 In
1 select * from score where degree in (85,86,88)
2 Linq:
3 from s in Scores
4 where (
5 new decimal[]{85,86,88}
6 ).Contains(s.DEGREE)
7 select s
8 Lambda:
9 Scores.Where( s => new Decimal[] {85,86,88}.Contains(s.DEGREE))
6.or 条件过滤
1 查询Student表中"95031"班或性别为"女"的同学记录。
2 select * from student where class ='95031' or ssex= N'女'
3 Linq:
4 from s in Students
5 where s.CLASS == "95031"
6 || s.CLASS == "女"
7 select s
8 Lambda:
9 Students.Where(s => ( s.CLASS == "95031" || s.CLASS == "女"))
7.排序
1 以Class降序查询Student表的所有记录。
2 select * from student order by Class DESC
3 Linq:
4 from s in Students
5 orderby s.CLASS descending
6 select s
7 Lambda:
8 Students.OrderByDescending(s => s.CLASS)
8.count（）行数查询
1 select count(*) from student where class = '95031'
2 Linq:
3 ( from s in Students
4 where s.CLASS == "95031"
5 select s
6 ).Count()
7 Lambda:
8 Students.Where( s => s.CLASS == "95031" )
9 .Select( s => s)
10 .Count()
10.avg()平均
1 查询'3-105'号课程的平均分。
2 select avg(degree) from score where cno = '3-105'
3 Linq:
4 (
5 from s in Scores
6 where s.CNO == "3-105"
7 select s.DEGREE
8 ).Average()
9 Lambda:
10 Scores.Where( s => s.CNO == "3-105")
11 .Select( s => s.DEGREE)
11.子查询
1 查询Score表中的最高分的学生学号和课程号。
2 select distinct s.Sno,c.Cno from student as s,course as c ,score as sc
3 where s.sno=(select sno from score where degree = (select max(degree) from score))
4 and c.cno = (select cno from score where degree = (select max(degree) from score))
5 Linq:
6 (
7 from s in Students
8 from c in Courses
9 from sc in Scores
10 let maxDegree = (from sss in Scores
11 select sss.DEGREE
12 ).Max()
13 let sno = (from ss in Scores
14 where ss.DEGREE == maxDegree
15 select ss.SNO).Single().ToString()
16 let cno = (from ssss in Scores
17 where ssss.DEGREE == maxDegree
18 select ssss.CNO).Single().ToString()
19 where s.SNO == sno && c.CNO == cno
20 select new {
21 s.SNO,
22 c.CNO
23 }
24 ).Distinct()
12.分组 过滤
1 查询Score表中至少有5名学生选修的并以3开头的课程的平均分数。
2 select avg(degree) from score where cno like '3%' group by Cno having count(*)>=5
3 Linq:
4 from s in Scores
5 where s.CNO.StartsWith("3")
6 group s by s.CNO
7 into cc
8 where cc.Count() >= 5
9 select cc.Average( c => c.DEGREE)
10 Lambda:
11 Scores.Where( s => s.CNO.StartsWith("3") )
12 .GroupBy( s => s.CNO )
13 .Where( cc => ( cc.Count() >= 5) )
14 .Select( cc => cc.Average( c => c.DEGREE) )
15 Linq: SqlMethod
16 like也可以这样写:
17 s.CNO.StartsWith("3") or SqlMethods.Like(s.CNO,"%3")
13.分组
1 查询Score表中至少有5名学生选修的并以3开头的课程的平均分数。
2 select avg(degree) from score where cno like '3%' group by Cno having count(*)>=5
3 Linq:
4 from s in Scores
5 where s.CNO.StartsWith("3")
6 group s by s.CNO
7 into cc
8 where cc.Count() >= 5
9 select cc.Average( c => c.DEGREE)
10 Lambda:
11 Scores.Where( s => s.CNO.StartsWith("3") )
12 .GroupBy( s => s.CNO )
13 .Where( cc => ( cc.Count() >= 5) )
14 .Select( cc => cc.Average( c => c.DEGREE) )
15 Linq: SqlMethod
16 like也可以这样写:
17 s.CNO.StartsWith("3") or SqlMethods.Like(s.CNO,"%3")
14. 多表查询
1 select sc.sno,c.cname,sc.degree from course as c,score as sc where c.cno = sc.cno
2 Linq:
3 from c in Courses
4 join sc in Scores
5 on c.CNO equals sc.CNO
6 select new
7 {
8 sc.SNO,c.CNAME,sc.DEGREE
9 }
10 Lambda:
11 Courses.Join ( Scores, c => c.CNO,
12 sc => sc.CNO,
13 (c, sc) => new
14 {
15 SNO = sc.SNO,
16 CNAME = c.CNAME,
17 DEGREE = sc.DEGREE
18 })
19 .Average()
